Extensibility in iOS 8: Explained

Extensibility, one of the new features coming to iOS 8 this fall, is one of the most significant advancements since the launch of the App Store in 2008. With Extensibility, apps can project widgets into Notification Center's today view, provide custom upload and update functionality, and custom actions in Share Sheets, hook filters into the Photos app, provide custom keyboards system-wide, and access your files anywhere via iCloud Drive or third-party document providers like Dropbox or Google Drive. And they can do all this while maintaining the high level of security built into iOS. So, who do they all work?